Coverings and insulations for electric wires and cables are conventionally formed using thermoplastic resins, such as olefin resin, which have outstanding extrudability and electrical insulating properties. Recently, there has been an increasing demand for flame-retarded versions of these materials.
In general, these thermoplastic resins are flame-retarded by being mixed with compounds which contain halogens such as chlorine and bromine.
Insulation or sheath materials for those electric wires or cables used in vehicles and nuclear power stations are required to have a flame resistance high enough to withstand, for example, the VW-1 flame test of UL Subj. 758, vertical flame test of ICEA S-61-402, and vertical tray flame test of IEEE std. 383. For their tensile properties, these materials are expected to have the tensile strength of 1.0 kg/mm.sup.2 or more and an elongation percentage of 350% or more, as provided for polyethylene by the JIS. Also, the materials are standardized with respect to the rate of production of hydrochloric acid gas. For cables used in nuclear power stations, in particular, the halogen gas production rate of the materials in combustion must be less than 100 mg/g.
A resin composition containing the aforementioned halogen-contained compounds certainly has a satisfactory flame resistance and other good properties. When it burns, however, the composition produces plenty of black smoke containing gases which are harmful to health or tend to corrode metal and the like.
Conventionally known is a method for avoiding such an awkward situation. According to this method, the thermoplastic resins are mixed with metal oxide hydrates, such as aluminum hydroxide and magnesium hydroxide, for use as flame retardants, which are very low in smoking property, harmfulness, and corrosiveness.
The use of the metal oxide hydrates, however, entails the following problems to be solved.
First, the thermoplastic resins should be mixed with a large quantity of the metal oxide hydrates, in order to withstand, for example, the VW-1 flame test provided by UL Subj. 758. If these inorganic materials are compounded in plenty, however, the resulting resin composition will be lowered in mechanical and electrical properties. In particular, the tensile properties will be considerably lowered.
Secondly, these metal oxide hydrates produce the flame retarding effect by causing dehydration. However, the dehydration suddenly occurs within a specific temperature range, so that it can contribute to only part of a series of reactions, including heating, fusion, decomposition, and ignition, before the firing of the polymer, and after all, the resulting resin composition is liable to fail to withstand the VW-1 flame test. If the metal oxide hydrates are compounded in plenty, in particular, the obtained resin composition may certainly be improved in oxygen index, as a yardstick of the flame resistance. However, ash produced by the combustion, which serves to restrain the fusion of the polymer, is too soft to keep its shape, so that the composition cannot withstand the VW-1 flame test.
Thirdly, if magnesium hydroxide is used as a flame retardant for a cable covering or sheath, for example, it will react with carbon dioxide in the ambient air, thereby producing magnesium carbonate, which will be separated on the surface of the covering or sheath, thus entailing chalking. Although this chalking effect has no bad influence upon the properties of the cable covering or sheath, it spoils the external appearance of the product and lowers its commercial value.